This invention generally relates to a method and apparatus for synchronizing a facsimile receiver with a facsimile transmitter and more particularly relates to the synchronization of the speed and phase of the document scanning and copy reproduction devices in a facsimile transmission system.
To effect the reproduction of an acceptable copy of the transmitted document, both the transmitter and receiver should be maintained in synchronization. Not only should the document and copy frames be in synchronization, that is, the received copy page should start at the appropriate time, but additionally, throughout transmission, the data samples must be synchronized so that they occur with respect to the copy at locations corresponding to their positions on the transmitted document. A particular system, as described in U.S. Pat. No. 3,670,105 by Houck issued June 13, 1972 and entitled "Phasing System for Facsimile Recorders," provides phase synchronization of the receiver at the start of the transmission; however, such synchronization may be lost during transmission and without corrective steps being taken could result in unacceptable copy. Various systems have been suggested utilizing commercially available power source frequencies as synchronizing signals. The use of such synchronization signals are not acceptable for use in high resolution systems. At remote locations the power source frequencies and phase may vary resulting in cumulative errors. For example, should the transmitting power frequency be greater than the received frequency, a noticeable quantity of information would be lost due to the inability of the receiver to print at the transmitting speed. Additionally, with respect to each high resolution system, phase variations between the frequency of the transmitter power source and the receiver power source, during transmission, could produce visible distortion patterns in the received copy such as "wood graining." It is further desirable that initial and continued synchronization occur in a short time period since facsimile systems frequently use long distance telephone facilities which are costly.
The device hereinafter described provides synchronization between a facsimile transmitter and receiver with respect to both frequency as well as phase throughout the entire transmission. A receiving copy drum is initially synchronized with a transmitter document drum and maintained in synchronization throughout transmission. One illustrated embodiment utilizes synchronous motors at both the transmitting and receiving terminals to drive the document and copy drums. Such motors are particularly adapted for this application since their speed may be controlled or varied by the frequency of the electrical energy applied to them. An alternate embodiment utilizes direct current motors in a frequency controlled phase-lock loop thus providing the desired motor speed and position control. Speed and position synchronization between the transmitting and receiving terminals in accordance with the illustrated embodiments may be conveniently provided by solid state logic devices thus providing relatively economical implementation.